The broad objective of this research project is to elucidate the factors that influence transmitter release from the motor nerve terminal. It is hoped that this project will lead to a better understanding of neuromuscular and synaptic transmission. The project will examine the role of presynaptic muscarinic and nicotinic receptors on the motor nerve terminal. It has been proposed that these autoreceptors may regulate the release of transmitter from the motoneurons. The project will examine the effect of nicotinic and muscarinic antagonists and muscarinic agonists on neuromuscular transmission in the rat. Intracellular recording techniques will be used to monitor miniature end-plate potentials and end-plate potentials in the isolated rat diaphragm phrenic nerve preparation. Analysis of transmitter release will include measurements of the amount of transmitter release with each evoked stimulus (quantal release) and replenishment of transmitter in the nerve terminal during repetitive neural stimulation (mobilization). To date the role of presynaptic autoreceptors in regulating transmitter release is uncertain. Some studies support a positive feedback role for these receptors while other investigators support a negative feedback role, and still others propose opposite roles for muscarinic versus nicotinic autoreceptors. While biochemical and muscle contraction methodologies are important in elucidating the role of these receptors, an electrophysiological approach is also essential. Some of the discrepancies in interpretation of results may be due to limitations of the method used and choice of drug concentration. Additional studies are needed before firm conclusions can be made about the specific actions of the presynaptic autoreceptors. The project will examine the impact of bath applied nicotinic and muscarinic antagonists in a range of concentrations to determine if high concentrations may produce effects that diverge from the effects observed with lower concentrations. Blockade of postsynaptic nicotinic receptors at higher concentrations may contribute to the reported presynaptic changes. It is hoped that this project will not only clarify the role of presynaptic receptors but also lead to means of controlling transmitter release. This could be of benefit for the treatment of neurological and neuromuscular diseases.